1. Field of the Invention
The present invention relates to a communication control unit provided in a computer system for increasing its efficiency, a communication control system consisting of multiplexed communication control units, and a method for switching communication control units.
In general, when an active unit in a duplex configuration of communication control units of a computer system breaks down, the active unit is switched to a stand-by unit based on some sort of error information. It is desirable that the units be switched quickly and without any interruption of the communications.
2. Description of the Related Art
As will be explained in detail later with reference to the drawings, conventional duplex configuration computer systems are provided with an active communication control unit (CCU) and a stand-by communication control unit (CCU) connected with each other in a central processing unit (CPU) in the computer.
In such a computer system, when trying to detect conditions disabling communications for some reason other than hardware abnormalities, the CPU has to use a timer for monitoring or perform polling processing on the active communication control unit so as to detect if the communication control unit fails to make a response or makes an abnormal response.
With monitoring using a timer, however, it is difficult to determine the monitoring time. If the time is set too long, detection of abnormalities will be delayed, while conversely if set too short, a problem occurs of erroneous detection in the middle of processing a large amount of data. Further, when detecting an abnormality by polling processing, there is the problem that the excess load due to the polling processing ends up burdening both the CPU and the active communication control unit. Further, it was not possible to confirm the validity of the communication function itself either with the timer monitoring or the polling processing.